Plotters and scanners are well known in the art, as are internal drum plotters. The Dolev plotter, manufactured by Scitex Corporation Ltd. of Herzlia, Israel, is one example of an internal drum plotter. A schematic side view of an internal drum plotter is shown in FIG. 1.
Internal drum plotters typically comprise a cylindrical element 10, onto the inner surface of which is placed a medium 11 to be exposed. Inside of cylindrical element 10 is an optical system for bringing a light beam, labeled `A`, from a light source (not shown) onto the inner surface of cylindrical element 10. In most internal drum plotters, the optical system minimally comprises a lens (not shown) for focusing the light beam and a rotatable mirror or prism 12, which rotates about a central axis 14 of the cylindrical element 10, for directing the light beam to the desired location.
Thus, if the optical system only rotates, the light beam A marks a circular path along the inner surface of cylindrical element 10 and in that manner, exposes the medium 11. However, mirror 12 is typically translated while rotating and thus, light beam A scans along a spiral path.
It is often desired to increase the operating speed of the internal drum plotter. In the prior art, this has been implemented in one of three ways:
(a) increasing the rotation speed of the mirror 12;
(b) providing multiple beams, such as beams A and B of FIG. 1, to mirror 12; or
c) a combination of (a) and (b).
The first solution is limited by the quality and characteristics of available motors and bearings since, among other problems, the mirror 12 is rarely perfectly balanced and the better quality motors and bearings which are operative to rotate mirror 12 in a balanced manner are very expensive.
The second solution requires that each beam write a different line of the medium 11. Thus, in the system of FIG. 1, beam A exposes a first, leftmost line and beam B exposes a second, rightmost line, as shown in the upper portion of the Figure. After the mirror 12 rotates a full 180 degrees, as shown in the righthand portion of the Figure, the beams A and B are in opposite positions and thus, beam A exposes the rightmost line and beam B exposes the leftmost line.
Since the above scenario is unacceptable, prior art internal drum plotters utilizing multiple beams either use a flat mirror and only expose 180 degrees of the cylindrical element or they incorporate an image rotating system, such as a dove prism, into the system. The latter solution adds another optical element into the system which adds complexity to the system, especially since a dove prism additionally has to be rotated half as fast as the mirror 12.
Use of more than one scanning beam for scanning a photosensitive film is known for external drum and flat bed scanners. Methods for generating a plurality of scanning beams and employing them in external drum and flat bed applications are described in the following documents, the disclosures of which are incorporated herein by reference:
Technical Report #384, issued in August 1978 by Isomet Corporation, P.O.B. 1634, Port Royal Rd., Springfield Va., USA, and entitled: "Technical report on multibeam laser scanning";
"Applications of multi-beam acousto-modulators in laser-electrophotographic printing and drawing machines", Tokes, S. B., SPIE Proceedings No. 397, 1983;
U.S. Pat. No. 4,577,932 to Gelbart; and
Israel Patent Application 100635, assigned to the common assignees of the present invention.
U.S. Pat. No. 4,591,242 to Brookman et al. describes an optical scanner having multiple simultaneous scan lines with different focal lengths. The optical scanner comprises a plurality of lasers and a common optical element through which the entirety of beams from the lasers pass. Each laser beam is associated with its own optical element which focuses the beam at its unique distance from the common optical element.
U.S. Pat. No. 4,289,371 to Kramer describes an optical scanner using plane linear diffraction gratings on a rotating spinner.